In an inductively coupled plasma (ICP) emission spectrometer, a plasma-generating gas (e.g. argon) is ionized by an electromagnetic field created by supplying radio-frequency power to an induction coil. While the obtained plasma is maintained by the electromagnetic field, a sample atom is introduced into the plasma. The sample atom is excited by the plasma, and when the excited atom returns to a lower energy level, it emits light whose wavelength is specific to the atom. By performing a spectrometry of this light, a qualitative and quantitative determination of the sample is performed.
In an ICP emission spectrometer, in order to supply radio-frequency power to the plasma, a configuration is commonly used in which an LC resonance circuit formed by an induction coil and a capacitor is driven by a radio-frequency power source which supplies, for example, radio-frequency power of several hundred watts to several kilowatts at a frequency of 27 MHz. To produce an oscillation with high efficiency in such a radio-frequency oscillation circuit, the load impedance seen from the radio-frequency power source should preferably be constant, and furthermore, the impedance should be matched with the optimum load impedance of the power source. However, when plasma is generated by passing a radio-frequency current through the induction coil, the impedance of the induction coil changes due to the effect of the induction current caused by the movement of charged particles in the plasma. The impedance of the induction coil also changes with a change in the state of the plasma, which can occur depending on the state of the plasma-generating gas or that of the sample to be analyzed, the amount of power supplied to the plasma, and other factors. Such a change in the impedance of the induction coil leads to a change in the load impedance seen from the radio-frequency power source, causing the impedance matching to deviate from the optimum state.
To overcome this problem, a self-oscillating circuit is commonly used, in which the LC resonance circuit formed by the induction coil and the capacitor is driven by a switching circuit, such as a half-bridge or full-bridge circuit including switching elements, with a positive feedback of an electric current from the LC resonance circuit to a control terminal of the switching element via a transformer or the like (see Patent Literatures 1-3). In such a self-oscillating circuit, when the impedance of the induction coil changes depending on a change in the state of the plasma, the oscillation frequency of the LC resonance circuit automatically changes. As a result, the load impedance seen from the switching circuit is constantly maintained at optimum levels, allowing the oscillation to continue with high efficiency without requiring any special control or command from the outside.
In a self-oscillating circuit commonly used in low-power applications, when a power supply to the radio-frequency LC oscillation circuit is started by energizing a power source, the small amount of noise in the DC power source or oscillation circuit is amplified by the positive feedback loop, whereby an oscillation is started and sustained. While the principle of sustaining the oscillation in a self-oscillating radio-frequency oscillation circuit for the previously described plasma generation is also the same, the situation is different when starting the oscillation. This is due to the following fact: In the self-oscillating circuit for low-power applications, the switching element, such as MOSFET, is driven in an operation range comparatively close to the linear range and the ON/OFF operation can be easily started, whereas in a self-oscillating circuit for producing a high power of several kilowatts, the switching element is driven in an operation range close to the saturation range and high power is needed to start the ON/OFF operation. Therefore, in many cases, merely energizing the power source does not immediately lead to the state of oscillation. This means that some device for starting the oscillation circuit is necessary.
To address these problems, the radio-frequency oscillation circuits described in Patent Literatures 1-3 have adopted a technique in which a DC bias circuit is connected to the control terminal of the switching element; after the main power source is energized, the power current is monitored and the DC bias voltage is gradually increased until the onset of an oscillation is detected. However, if the DC bias voltage is increased to an excessively high level, the switching element will be completely turned on and will allow a considerable amount of current to pass through, which may possibly damage the element. Therefore, the DC bias voltage should be controlled according to the gain, input-voltage threshold and other properties of the switching element to prevent the voltage from being excessively high. However, since those properties considerably vary from one element to another, a complex control is needed to start the radio-frequency oscillation in a stable manner.
Besides, in a configuration which drives the LC resonance circuit by a full-bridge circuit as described in Patent Literature 2, the DC bias circuit needs to be provided for each of the four switching elements. Furthermore, two of the four DC bias circuits must be a circuit floated from the ground potential, which means the device cost will be considerably high.
As another startup method, it is possible to connect a switching element dedicated to startup parallel to the switching element which operates during the steady-state oscillation, to compulsorily start the oscillation by driving the switching element dedicated to startup only during a certain period of time in the starting phase. According to this startup method, the previously described problem due to the addition of the DC bias circuit does not occur. However, connecting the switching element dedicated to startup parallel to the main switching element causes an equivalent increase in the output capacitance of the switching elements. Since the frequency characteristics of a switching element significantly depends on its output capacitance, the increase in the output capacitance deteriorates the frequency characteristics and lower the gain in the radio-frequency range, making it difficult to ensure an adequate amplitude of oscillation.